I have replaced the original lap/shoulder belts in several 67-74 era vehicles with 3-point retractors. They eliminate all the aggravation of those original style shoulder belts.
You could install 3-point retractor systems for the two outboard front seat positions for your Buick. They will bolt into the original attachment points and are designed for your vehicle structure. Use matching lap belts for the other seating positions. If you have a Skylark, you can use one of the available Chevelle kits to add 3-point harnesses for the outboard rear seating positions, if you desire. (I used Morris Concepts restraints for my 68 GTO, including retractors in the rear outboard positions)
You can buy them from a variety of suppliers. Here are a few:
Seat Belts Plus:
Seat Belt Planet:
Beam's Seat Belts:
Morris Classic Concepts:
You can also order these from companies like Summit Racing or Speedway.
All these seat belt suppliers offer a wide variety of colors combined with either lift latch or push-button style latches. Most also offer an OEM appearing "GM" pushbutton option.
Choosing NEW belts provides an added margin of protection; I will not trust my life - or my family - to 50 year old seat belt webbing.
I installed Morris Concepts belts in my GTO and in my 69 C-10 pick-up.
My 58 Buick has lap belts with chrome lift latch buckles from Beams (purchased from Cars Inc in Michigan)
Make sure you are buying MVSS 201/210 compliant belt systems.